Gas chromatography (GC) is generally performed using one or more columns to separate a sample of material into its constituent components. In conventional comprehensive two-dimensional gas chromatography, referred to as comprehensive GC-GC or GCxGC, two columns of different selectivity are linearly coupled to provide enhanced separation of complex chemical mixtures that cannot be realized by either column alone. Examples of complex matrices include fuels (e.g., gasoline, jet fuel, kerosene, diesel fuel), oils (e.g., crude oil, lubricating oils), extracts from environmental samples (e.g., air, water, and soil), food extracts and clinical samples (e.g., blood, plasma, urine, tissue extracts).
In prior GCxGC analysis systems, a one-stage or a two-stage modulator located between the primary column and the secondary column transfers the effluent from the first column to the second column. In a two-stage modulator the first stage of the modulator collects the effluent from the primary column and the second stage of the modulator transfers, or injects the collected effluent to the secondary column. Two stage modulators are typically designed as thermal or flow-switching modulators.
A thermal two-stage modulator typically uses a cryogenic material to cool the effluent in the first stage prior to transfer to the second stage. The second stage of the modulator heats the effluent prior to injecting the effluent into the secondary column. Cooling the effluent concentrates the chromatographic peaks, which improves the analysis in the secondary column. Unfortunately, thermal two-stage modulators are complicated to design and manufacture, and require the use of a cryogenic material to cool the effluent from the primary column.
A flow-switching modulator is typically less complex than a thermal two-stage modulator and does not require a cryogenic material. However, a flow-switching two-stage modulator requires the use of complex fluid couplings that must be carefully and precisely dimensioned and scaled. These requirements lead to limitations of the GC column dimensions and require reconfiguring the components and couplings of the modulator when changing columns and/or operating conditions. Flow switching also requires higher than optimum flow rates in the secondary column(s) and places limitations on the flow ratio between the primary column and the secondary column.
In a prior one-stage modulator, a fast acting diaphragm valve transfers the effluent from the primary column to the secondary column. While a diaphragm valve modulator is relatively simple to fabricate and operate, it suffers from other limitations. For example, a diaphragm valve modulator cannot be operated at temperatures exceeding approximately 250 degrees Celsius (C). This restricts the use of such a modulator to lower boiling samples. Further, mechanical valves, such as diaphragm valves, are subject to transient, temporary pressure and flow fluctuations immediately after actuation. Depending on the dimensions of the fluid couplings in the system, this can interfere with the precise transfer of the sample from the primary column to the secondary column. Further, in such a diaphragm valve, the sample material passes through the valve body, where the sample may come into contact with active or adsorptive sites from the valve material or material used to construct the valve. These sites may contaminate the sample and skew the analysis.
Therefore, it would be desirable to have the ability to efficiently perform comprehensive two-dimensional GC analysis.